FIG. 1 is an illustration of a screen printer disclosed in my above-identified co-pending application. The printer uses a vacuum pull-down method for deflecting a printing screen 502 which is coated with viscous ink, into contact with a substrate 100. In the preferred embodiment the substrate is a printed circuit board substrate and the ink comprises conductive metal particles suspended in a viscous carrier or screening agent. It should be understood, however, that other types of substrates and inks may be used with this printer (all such materials are collectively referred to hereafter as "ink").
Printing screen 502, in which a desired printing pattern has been formed by conventional means, is supported in a screen holder 504. Holder 504 is mounted at either side to frame mount arms 506 and 508 by support members 510 and 512 and bolts 737 and 738. Frame mount arms 506 and 508 are fixably mounted at one end to pivot bar 518. Pivot bar 518 extends through a pair of positioning devices 520 and 522, described hereinafter, which are used to adjust the position of pivot bar 518 along the Y-axis and around the Z-axis (commonly referred to as the "theta axis"). Positioning devices 520 and 522 are manually controlled, respectively, by knobs 534 and 536. Adjusting knob 772 is used to adjust the position of screen holder 504 along the X-axis. Thus, by turning knobs 534, 536 and 772, the operator can adjust the position of screen holder 504 until screen 502 is precisely registered above substrate 100.
A pair of counterweights 524 and 526 are rigidly attached to pivot bar 518, allowing the operator to easily rotate screen holder 504 about pivot bar 518. Alternatively, air springs may be used for this purpose. The operator does so by grasping handle 528 and either lifting it until the screen holder is in the fully opened position or pushing it downward until the bottom edges of the screen holder contact vacuum seal 678 inserted into the upper surface of support frame 530. Support frame 530 has a central recess therein, in which substrate 100 is positioned on top of printing platen 556. Platen 556 is bonded to a product carrier 550 (illustrated in FIG. 2). Product carrier 550 and, hence, platen 556 may be adjusted along the Z axis by rotating knobs 708A and 708B, to thereby space substrate 100 from screen 502 to the desired degree. Platen 556 has regularly spaced apertures 558 therein used for substrate hold-down, as described below. Non-porous tape 557 (FIG. 2) is used to cover the apertures in platen 556 not covered by the substrate.
When screen holder 504 is rotated to the fully down position, the rear section thereof depresses a micro-switch 529, which causes a vacuum to draw substrate 100 into firm contact with platen 556, thereby preventing relative movement between them during the printing operation. In the down position the surface of screen 502 is essentially parallel to the surface of substrate 100. After ink has been applied to the upper surface of screen 502, filling all of the unmasked screen openings, a vacuum print system is energized. This system evacuates the region between substrate 100 and screen 502, causing screen 502 to deflect downward into contact with substrate 100 and ink to be transferred through the screen openings onto the substrate.
After ink transfer, the vacuum is released and the operator rotates screen holder 504 away from substrate 100, removes the substrate from the printer and inserts the next substrate for printing. Concurrent with release of the print vacuum, a pilot actuated valve is operated to vent the print chamber to atmospheric pressure, thereby avoiding a double strike of ink onto the substrate. Upon rotating screen holder 504 away from substrate 100, micro-switch 529 is opened, shutting off the hold-down vacuum and returning the pneumatic hold-down system 900 to the standby mode. Screen 502 may be removed from holder 504 whenever the operator desires to do so, such as at the end of a production run.
FIG. 2 depicts a sectional view of screen holder 504, support frame 530 and the hold-down mechanism for a substrate 100. A product carrier 550 is positioned in a recess of the support frame. The recess periphery is defined by the vertical portions of side members 705 and 707, a front member and a rear member (not shown in detail), while the recess bottom is defined by support member 531. The lateral dimensions of product carrier 550 are slightly less than the corresponding dimensions of the recess, thereby defining an evacuation channel 552 between product carrier 550 and recess.
Screen holder 504 employs an extruded channel 660 having a U-shaped cross-section. In the preferred embodiment, channel 660 is made of aluminum. Four sections of such channel stock are welded together into a rectangular frame, as shown in FIG. 1. In FIG. 1, screen holder 504 is in a partially open position, while in FIG. 2 it is in the fully down position.
A tensioning bar 662, shown in cross-section in FIG. 2, runs the length of channel 660 and is secured to the top portion 664 of U-shaped channel 660 by a plurality of securing bolts 666. Securing bolts 666 are fed through a plurality of bores spaced symmetrically about channel 660. A plurality of tension adjusting bolts 668 are secured to tensioning bar 662 through outside leg 675 of U-shaped channel 660. Again, bolts 668 are spaced symmetrically along the length of channel 660. Tensioning bar 662 has an offset portion 670 which has two legs 672 forming a U-shaped recess 673 that accepts a tensioning rod 674 therein. The inner sides of legs 672 are tapered inwardly so that the entrance to recess 673 is slightly wider than the diameter of rod 674 combined with a single wrap of screen 502, while the bottom of recess 673 is slightly narrower than the rod 674/screen 502 combination, so that when rod 674 is inserted therein it is held snugly. In the preferred embodiment tensioning rod 674 is made of stainless steel and may be inserted along the entire length of channel 660.
Printing screen 502 passes over the inside leg 676 of channel 660, over leg 672 and is secured in recess 673 by tensioning rod 674. Inside leg 676 has a rounded bottom 677 and is slightly longer than outside leg 675. Screen 502 is stretched taut by adjusting bolts 666 around the periphery of channel 660. Inside leg 676 is rounded to reduce the likelihood of tearing screen 502 where the two come in contact. Support frame 530 has a channel 680 spaced around the periphery of the recess, as shown more clearly in FIG. 7a of my aforesaid copending application.
Referring again to FIG. 2, the seal 678 (in Figure) is inserted into channel 680, so that when U-shaped channel 660 is in the fully down position, rounded bottom 677 of inner leg 676 rests on the seal. In the preferred embodiment seal 680 is made of very low durometer rubber (e.g., 50-60 DUR). Since rounded bottom 677 is pulled downward into the seal by vacuum pressure during the printing operation, slightly shorter outside leg 675 rests against the top surface of support frame 530 when screen holder 504 is in the fully-down position.
As indicated above, screen 502 has been treated in conventional manner before insertion into holder 504 so that the openings therein define the desired pattern to be printed on substrate 100. When ink of sufficient viscosity is applied to the upper surface of screen 502, these openings remain covered prior to printing. In the preferred embodiment, the ink has a minimum viscosity of approximately 200,000 centipoise. Thus, when screen 502 is tautly mounted in screen holder 504, as described above, and holder 504 is in the fully-down position, two parallel air-tight paths are defined, one for screen hold-down and the other for substrate hold-down during the printing operation.
Referring again to FIG. 2, the screen hold-down path begins with the region 555 between screen 502 and platen 556, and continues along evacuation channel 552 to evacuation port 604, through fitting 606, hose 608, and a fitting (at 613 in my aforesaid copending '049 application at FIG. 8) to vacuum hold-down system (at 900, shown in FIG. 12 of my aforesaid copending '049 application). When a vacuum is drawn along this path, screen 502 is deflected downward into contact with substrate 100, and ink on the upper surface of screen 502 is drawn through the openings therein into contact with substrate 100. When the vacuum is released, screen 502 returns to its normal unstretched position, but a quantity of ink is transferred to the surface of substrate 100 through a combination of vacuum and meniscus forces, thereby defining the desired pattern on substrate 100.
The substrate hold-down path begins with apertures 558 in platen 556, and runs through channels 554 and manifold grooves in product carrier 550, hollow shafts 584 and 586 in vertical adjusting bolt 564, bore 588 in vertical adjustment mount bracket 567, fitting 578 and hose 591 to vacuum system 900. The vacuum drawn through this path holds substrate 100 firmly in contact with platen 556 during the printing operation so that it does not move in response to meniscus forces when screen 502 returns to its unstretched rest position.
It is important to maintain the viscosity of the conductive ink at predetermined levels to ensure adequate flow through the screen 502 during printing. If, however, suitable conductive inks or slurries are improperly stored prior to use, solvents or binders within the inks may be subjected to evaporation which dries the ink and changes the viscosity to possibly unacceptable levels. Additionally, if the inks are stored in an oxygen containing environment, oxidation of the conductive metals in the ink is likely to occur which may disadvantageously prevent complete fusing of the ink to the substrate at controlled temperature.
It is accordingly one object of the present invention to provide packaging for conductive slurries or inks which allow for long shelf life of the inks prior to use.
Another object is to maintain the viscosity of the ink at predetermined levels to ensure adequate flow through a screen during printing by preventing evaporation of volatiles and other components in the ink.
Still another object is to store the ink out of contact with oxygen to prevent oxidation of conductive metals in the ink.
A further object is to store ink in packaging which is easily attachable to a screen print frame for easy and immediate use.